Concise string representations of index sets, like [4-5, 16, 19-21] or alternatively [4, 5, 16, 19, 20, 21]

NSIndexSet+DHDescription.m

static NSString * DHStringFromRange(NSRange range);

@implementation NSIndexSet (DHDescription)

- (NSString *)dh_description
{
	return [self dh_descriptionShowingRanges:YES];
}

- (NSString *)dh_descriptionShowingRanges:(BOOL)shouldShowAsRanges
{
	static NSString * const separator = @", ";
	NSUInteger const firstIndex = [self firstIndex];
	NSMutableString * const description = [NSMutableString stringWithString:@"["];
	
	if (shouldShowAsRanges) {
		[self enumerateRangesUsingBlock:^(NSRange range, BOOL *stop) {
			if (range.location != firstIndex) [description appendString:separator];
			[description appendString:DHStringFromRange(range)];
		}];
	}
	else {
		[self enumerateIndexesUsingBlock:^(NSUInteger idx, BOOL *stop) {
			if (idx != firstIndex) [description appendString:separator];
			[description appendString:[NSString stringWithFormat:@"%lu", (unsigned long)idx]];
		}];
	}
	
	[description appendString:@"]"];
	
	return description;
}

@end

static NSString *DHStringFromRange(NSRange range)
{
	switch (range.length) {
		case 0:
			return @"";
		case 1:
			return [NSString stringWithFormat:@"%lu", (unsigned long)range.location];
		default:
			return [NSString stringWithFormat:@"%lu-%lu", (unsigned long)range.location, (unsigned long)(range.location + range.length - 1)];
	}
}